I just got the Surface 3 (non-pro), but I could see upgrading and keeping both around if the Pro 4 comes with a 14" display...
With a 12" screen it would be too close in size and form factor to my new Surface but for me a 14" screen size would put it in a whole other camp plus if it came with Skylake and had a good price, I have to say, I would be VERY tempted! My plan was to get the iPad Pro but I'm liking Windows 10 on my Surface 3 alot and I am weighing the benefits of each. The Surface Pro 4 would have a full blown desktop operating system on a tablet whereas the iPad would still be IOS...
The bigger the better is my thinking for this year partially because my next phone is most likely going to be a phablet. My turbo is 5.2 inches and it's already feeling small next to all the Samsungs I see people using out and about. Plus my favorite phone that I ever owned was the HTC One Max which was 5.9" if I'm not mistaken... But I digress.
My point is, the bigger the phone I'm using, the bigger I want my tablet to be. There's no point, for me anyway, to have a phone that's about the same size as my tablet. If I'm going to use both, they should fit into completely different categories. That's why I'm looking at large tablets now. And 14", IMO, is a very large tablet!